Small test tool for PBD::Transmitter thread-safety tests

Launch as
```
 ./session_utils/run ardour7-debug_transmitter
 ./session_utils/debug ardour7-debug_transmitter
```

session_utils/debug_transmitter.cc

@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) 2022 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <iostream>
+#include <cstdlib>
+#include <thread>
+
+#include "common.h"
+
+#include "pbd/error.h"
+#include "pbd/semutils.h"
+
+using namespace std;
+using namespace ARDOUR;
+using namespace SessionUtils;
+
+PBD::Semaphore sem ("sync", 0);
+
+void worker (int i)
+{
+	sem.wait ();
+	PBD::error << "Thread: " << i << endmsg;
+}
+
+int main (int argc, char* argv[])
+{
+	SessionUtils::init();
+
+	int n_thread = 0;
+
+	if (argc > 1) {
+		n_thread = atoi (argv[1]);
+	}
+
+	if (n_thread < 1 || n_thread > 512) {
+		n_thread = 16;
+	}
+
+	printf ("Starting %d threads\n", n_thread);
+
+	std::vector<std::thread> threads;
+	for (int c = 0; c < n_thread; ++c) {
+		threads.push_back (std::thread (worker, c + 1));
+	}
+
+	for (size_t c = 0; c < threads.size (); ++c) {
+		sem.signal ();
+	}
+
+	for (auto& th : threads) {
+		th.join ();
+	}
+
+	SessionUtils::cleanup();
+	return 0;
+}